1. Field of the Invention
The present invention relates generally to the field of integrated circuit fabrication, and more specifically to the use of dielectric anti-reflective coatings in integrated circuit fabrication processes.
2. Description of the Related Art
The semiconductor industry's drive toward increasingly smaller integrated circuit geometries has led to the use of photolithography processes that employ radiation with increasingly shorter wavelengths. More specifically, in order to achieve sufficient resolution, deep submicron (less than approximately 0.5 micron) applications require the use of deep ultraviolet (DUV) radiation with wavelengths of, for example, approximately 248 nanometers, as compared to G-line (approximately 436 nanometers) or I-line (approximately 365 nanometers) radiation.
However, the use of DUW radiation has led to problems. In particular, reflections from underlying layers of metals, metal silicides and polysilicon during exposure has led to problems such as higher CD variances from area to area, standing wave effects and the well-documented "footing" problem. The formation of standing waves reduces critical dimension control and causes large linewidtlh variations over device topography. These problems are exacerbated by the increased reflectivity exhibited by many materials in the presence of DUV radiation. These problems are particularly severe in areas where the reflective surface topography is "stepped" (where the reflective surface is rising or falling, such as at the edges of a gate stack).
These problems have led to the use of anti-reflective coatings. Anti-reflective coatings are typically deposited on a substrate below a photoresist layer to control reflection of DUV radiation off of surfaces below the anti-reflective coating and thereby minimize the problems associated therewith. While helpful, known anti-reflective coatings are not completely effective in controlling undesirable reflections.
One problem with the use of anti-reflective coatings is that they are typically opaque and have a high absorption value and index of refraction, while the materials above the anti-reflective coatings are typically transparent and have a lower absorption value and lower index of refraction. For example, a thin film of opaque anti-reflective coating is often used beneath a transparent inter-level dielectric layer. A dielectric anti-reflective coating, or DARC may be used beneath inter-level dielectric layers.
The problem with using an anti-reflective coating in this manner is that the interface between the bottom surface of the dielectric and the top surface of the anti-reflective coating causes a reflection due to the index of refraction mismatch. Thus, although the DARC may be totally effective in preventing any radiation penetrating its upper surface from being reflected off of surfaces below the DARC, some radiation will be reflected off of the DARC/dielectric layer interface. This reflected radiation causes the aforementioned problems.
A second problem associated with anti-reflective coatings is that, for various reasons not of concern here, the integrated circuit fabrication process is such that the thickness of the anti-reflective coating must be thinner than the thickness required to completely prevent radiation from reflecting off surfaces below the anti-reflective coating and back.
What is needed is an anti-reflective coating that reduces reflections from surfaces beneath the anti-reflective coating as well as reflections from the upper surface of the anti-reflective coating itself.